Abstract
This lecture details the problems encountered by the standard dark matter model. One of the main ones is that the fraction of baryons detected in galaxies is extremely low, always less than 20% of the universe's baryon fraction (the ratio of baryon mass to total matter is 17%). This fraction of baryons maximizes for galaxies like the Milky Way, but becomes very low for dwarf galaxies, and the smallest must even be completely black. How can baryons be ejected from galaxies? For the smallest, it could be star formation, or even the beginning of phenomena linked to the re-ionization of the universe, and for the most massive, the impact of active galactic nuclei is necessary, although not yet confirmed. There are, however, small dwarf galaxies around the Milky Way, dominated by dark matter, but which have retained a non-negligible fraction of baryons, in contradiction with the predictions of the Standard Model. Other problems are the cusps of dark matter radial profiles, which even when flattened by supernovae, keep coming back, making the profiles steeper and steeper. Another is the loss of angular momentum of baryons due to dynamic friction on dark matter, and the formation of galaxy disks that are too small. Some solutions have been put forward, such as self-colliding dark matter, with a non-zero elastic collision cross-section, or lukewarm dark matter to suppress the multitude of unobserved satellites, and so on.
